1. Field of the Invention
The present invention relates to a control apparatus for an in-cylinder injection spark-ignition internal combustion engine capable of fuel injection in a compression stroke and an intake stroke.
2. Description of the Related Art
In order to reduce harmful gas components discharged from spark-ignition internal combustion engines that are mounted in vehicles, or improve the fuel-efficiency of the engines, there have recently been proposed various gasoline engines of an in-cylinder injection type in which fuel is injected directly into a combustion chamber, in place of conventional manifold-injection engines.
In an in-cylinder injection gasoline engine, an air-fuel mixture with an air-fuel ratio near a stoichiometric air-fuel ratio is generated around a spark plug at the time of ignition by, for example, injecting fuel from a fuel injection valve into a cavity that is formed in the top face of a piston. Thus, the fuel can be ignited with use of a generally lean air-fuel ratio, so that the discharge amounts of carbon monoxide (CO) and hydrocarbon (HC) are reduced, and the fuel-efficiency for idle operation or low-load drive can be improved considerably.
In the in-cylinder injection gasoline engine of this type, the injection mode is switched between a compression-stroke injection mode (late injection mode) and an intake-stroke injection mode (early injection mode), depending on the operating state of the engine, that is, engine load.
When the engine is in low-load operation, for example, therefore, the fuel can be made to be injected during a compression stroke by selecting the compression-stroke injection mode. Accordingly, an air-fuel mixture with an air-fuel ratio near the stoichiometric air-fuel ratio can be formed around the spark plug or in the cavity, so that the fuel can be ignited satisfactorily even with use of a lean overall air-fuel ratio. When the engine is in medium-or high-load operation, on the other hand, the fuel can be made to be injected during an intake stroke by selecting the intake-stroke injection mode. As in the case of a manifold-injection gasoline engine, therefore, the fuel can be burned in plenty by forming an air-fuel mixture with a uniform air-fuel ratio in the combustion chamber, so that an appropriate output can be secured for acceleration or high-speed drive.
The in-cylinder injection gasoline engine of this type, like the manifold-injection gasoline engine, is designed so that the ignition timing is corrected in accordance with its operating state or various operating conditions, such as cooling water temperature, in order to maintain a suitable state of combustion for the fuel.
In the case where the fuel injection mode is the intake-stroke injection mode, the correction of the ignition timing is carried out substantially in the same manner as in the case of the manifold-injection gasoline engine, without any problem.
In the case where the injection mode is the compression-stroke injection mode, however, a spray of the fuel injected into the combustion chamber must be ignited with a timing such that it is collected in the vicinity of the spark plug as the piston ascends, so that a set range for the ignition timing is very narrow. If the ignition timing for the compression-stroke injection mode is corrected in the same manner as that for the intake-stroke injection mode, therefore, ignition may be done when the fuel spray is not properly situated close to the spark plug, so that the combustion state may possibly be worsened. If the correction amount is particularly large, there is a possibility of a misfire taking place. Inevitably, in such a case, the operating state of the engine is not very stable, and the exhaust gas performance is lowered.